Lidar system

ABSTRACT

Methods and systems for using spectrally separated light pulses to collect more LIDAR information are presented. In one embodiment, a monochromatic pulse is transmitted to collect range information and a white pulse is transmitted a short time afterwards to collect spectral responsivity information or color of the target. In another embodiment, the white light pulse is used to collect both range and spectral responsivity information of the target. In another embodiment, the spectral separated laser is spatially spread in order to collect range information over more than one point at a time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to, U.S.application Ser. No. 11/903,302, filed on Sep. 21, 2007 entitled “LidarSystem,” which in turn claims the benefit of U.S. ProvisionalApplication No. 60/846,617, filed on Sep. 22, 2006, by Gregory C. Walshand titled “Full Color LIDAR System.” Each of those applications isincorporated herein by reference in their entireties.

BACKGROUND

The acquisition of data and subsequent generation of computer models forreal-world objects is of interest in many industries and for manyapplications including architecture, physical plant design,entertainment applications (e.g., in movies and games), surveying,manufacturing quality control, medical imaging, and construction, aswell as in cartography and geography applications. In order to obtainaccurate 3D models of an object, as well as the area in which thatobject exists in the real world, it is necessary to take accuratemeasurements or samplings of surfaces that make up the object and anyelements of the surrounding area. Historically, this sampling wascarried out using techniques that provided samples at the rate of tensor hundreds per hour at most.

Recent advances in scanning technology, such as technologies utilizingLIDAR scanning, have resulted in the ability to collect billions ofpoint samples on physical surfaces, over large areas, in a matter ofhours. In a LIDAR scanning process, the scanning device scans a laserbeam across a scene that encompasses the structure of interest and thebeam reflected from the scene is captured by the scanning device. Thescanning device thus measures a large number of points that lie onsurfaces visible in the scene. Each scan point has a measured locationin 3D space, to within some measurement error, that typically isrecorded relative to a point (x,y,z) in the local coordinate system ofthe scanner. The resulting collection of points is typically referred toas one or more point clouds, where each point cloud can include pointsthat lie on many different surfaces in the scanned view. LIDAR systemsare described, for example, in U.S. Pat. No. 5,988,862, issued on Nov.3, 1999, titled “Integrated System for Quickly and Accurately Imagingand Modeling Three Dimensional Objects,” which is hereby incorporatedherein by reference in its entirety to provide background informationregarding the present invention.

FIG. 1 shows an exemplary LIDAR scanning system 100. Scanning system 100utilizes a field digital vision (FDV) module 102 that includes ascanning device for scanning a target object 104, such as a building ora piece of machinery. The scanning device senses the position inthree-dimensional space of selected points on the surface of the object104. Based upon the light reflected back by the surface of the object104, the FDV module 102 generates a point cloud 106 that represents thedetected positions of the selected points. The point cloud 106 can alsorepresent other attributes of the detected positions, such asreflectivity, surface color, and texture, where desired.

A control and processing station 108 interacts with the FDV 102 toprovide control and targeting functions for the scanning sensor. Inaddition, the processing and control station 108 can utilize software toanalyze groups of points in the point cloud 106 to generate a model ofthe object of interest 104 that is stored in a database 118. A userinterface 116 allows a user to interact with the system 100, such as toview a two-dimensional (2D) representation of the three-dimensional (3D)point cloud 106, or to select a portion of the target object 104 to beviewed in higher detail. The processing and control station 108 caninclude any appropriate components, such as standard computer and/orprocessing components. The processing and control station 108 can alsohave computer code in resident memory, on a local hard drive or in aremovable drive or other memory device, which can be programmed to theprocessing station 108 or obtained from a computer program product suchas a CD-ROM or download signal. The computer code can includeinstructions for interacting with the FDV 102 and/or a user, and caninclude instructions for undertaking and completing any modeling and/orscanning process discussed, described, or suggested herein.

The FDV 102 can include an optical transceiver, shown in FIG. 1 as aLIDAR scanner 110, that is capable of scanning points of the targetobject 104, and that generates a data signal that precisely representsthe position in 3D space of each scanned point. The data signals for thegroups of scanned points can collectively constitute the point cloud106. In addition, a video system 112 can be provided, which in oneembodiment includes both wide angle and narrow angle CCD cameras. Thewide angle CCD camera can acquire a video image of the object 104 andprovides to the control and processing station 108, through acontrol/interface (C/I) module 114, a signal that represents theacquired video image. The acquired video image can be displayed to auser through the user interface 116 of the processing and controlstation 108. Through the user interface 116, the user can select aportion of the image containing an object to be scanned. In response touser input, the processing and control station 108 can provide ascanning control signal 120 to the optical transceiver 110 forcontrolling the portion of the surface of the object 104 that should bescanned by the transceiver 110.

The narrow angle CCD camera of the video system 112 can capture theintensity of light returned from each scan impingement point, along withany desired texture and color information, and can provide this capturedinformation to the processing and control station 108. The processingand control station 108 can include a data processing system (e.g., anotebook computer or a graphics workstation) having special purposesoftware that, when executed, instructs the data processing system toperform the FDV 102 control and targeting functions, and also to performthe model generation functions discussed elsewhere herein. Once theobject 104 has been scanned and the data transferred to the database118, the data and/or instructions relating to the data can be displayedto the user.

Conventional LIDAR scanning systems, such as the Leica HDS3000 systemand the Leica HDS4500 system, are monochromatic. That is, they generatedistance information based upon time-related measurements of the outputfrom a single wavelength laser. If any color information on the scannedobject or scene is required, it is typically obtained using a secondconventional, non-time resolved camera, as discussed above with respectto the FIG. 1 system 100. The auxiliary camera may be mounted inparallel (alongside, laterally displaced) with the LIDAR system orcoaxially by the use of either a beam-splitter or a separate movingmirror to intermittently intercept the LIDAR optical path. The two setsof data images, the LIDAR data and conventional camera data, may furtherbe combined using so-called “texture mapping” in which the non-timeresolved color information obtained from the conventional camera data issuperimposed upon the LIDAR data using dedicated software, so as toproduce a pseudo “color LIDAR” image.

This approach to “color LIDAR” can enhance the perception of the scannedobject or scene, but suffers from a number of disadvantages. Asdiscussed above, a second camera is required, adding to systemcomplexity and costs, and requiring additional system (camera-to-LIDAR)alignments and calibrations. The parallel systems also result inregistration errors (physical and software) and possible parallaxerrors, and often operate on the basis of different color and LIDARresolutions (pixel sizes, spot sizes). In brief, the result is a dualoptical system that is not only complex, but also suffers from generallyimperfect registration and mismatched optical resolutions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a scanning system that can beused in accordance with one embodiment of the present invention.

FIG. 2 is a block diagram illustrating an embodiment of a full colorLIDAR system in accordance with the present invention.

FIG. 3 is a block diagram illustrating an alternate embodiment of a fullcolor LIDAR system in accordance with the present invention.

FIG. 4 is a block diagram illustrating another alternate embodiment of aLIDAR system in accordance with the present invention.

FIG. 5 is a block diagram illustrating another alternate embodiment of aLIDAR system in accordance with the present invention.

FIG. 6 is a block diagram illustrating another alternate embodiment of aLIDAR system in accordance with the present invention.

FIG. 7 is a block diagram illustrating the combination of optical beamsutilizing a fiber optic coupler-splitter.

FIG. 8 is a block diagram illustrating the collimation and refocusing ofa launched and return laser beam, respectively, utilizing a single lens.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

FIG. 2 shows an embodiment of a full color LIDAR system 200 inaccordance with the present invention.

In the system 200 of FIG. 2, a pulsed laser 202 provides a laser pulsethat is propagated down an optical path, preferably a single modeoptical path 203, and divided by a 2× fiber optic splitter 204 into twopaths. A first path 206, a direct monochromatic “LIDAR” laser pulse, iscoupled directly to transmitter macro-optics 210, e.g. beam collimationoptics. A second path 208, the “illumination” path, is split off anddelayed through a comparatively long non-linear fiber thatwavelength-broadens the output to form a super-continuum or delayedwavelength-broadened “white” illumination laser pulse. A first LIDARoptical path 206 and a second illumination optical path 208 arerecombined by a second fiber optic coupler-splitter 209 and launched outof a common fiber exit of the coupler-splitter 209 towards themacro-optics 210. A monochromatic LIDAR pulse 206 forms the basis forthe timing/distance measurements made by the system 200; a delayedwavelength-broadened “white” illumination laser pulse 208 provides thebasis for the spectral/color measurements made by the system 200.Generation of a supercontinuum or “white” light pulse 208 is inaccordance with techniques well known to those skilled in the art usingreadily available off-the-shelf products. Typically, a Q-switched 532 nmmicrochip laser is propagated down non-linear fibers, e.g. photoniccrystal fibers.

The system 200 scans a combined direct monochromatic LIDAR pulse 206 andthe delayed wavelength-broadened “white” illumination pulse 208 across atarget object and receives a return light signal in the well knownmanner discussed above.

With continuing reference to FIG. 2, upon receipt of the return lightsignal through the receiver macro-optics 210, the monochromatic LIDARlaser pulse 206 is split off by a narrow-band filter 212, combined withauxiliary signals 213, which are used to determine the start of thelaser firing as well as calibration optical pulses, and directed towarda high-speed LIDAR detector 214, typically an avalanche photo diode(APD), for processing in the conventional manner. The output of theoptical detector 214 is provided to timing electronics 216 and then todetection circuits 218. That is, the monochromatic LIDAR pulse 206arrives at the LIDAR optical detector 214 well before the whiteillumination pulse arrives at the detection circuits 218. Thus, thetiming electronics 216 can detect the monochromatic LIDAR pulse arrivaland set the integration window for the detection circuits 218. Themonochromatic LIDAR pulse 206 is used in the conventional manner todetermine the range; the white illumination pulse 208 is used todetermine the spectral reflectivity as follows.

As further shown in FIG. 2, the remaining, substantially intact,broadband “white illumination” pulse 208 that has traveled from thereceiver side of the macro-optics 210 and though the narrow-band filter212 is further spectrally divided by a wavelength separator 220 into anumber of spectral color components λ₁, λ₂, λ₃. . . . Each of thespectral color components λ₁, λ₂, λ₃. . . is then propagated down aseparate route to a corresponding “color” detector1, detector 2,detector3 . . . , respectively, e.g. APDs, each of which is, in turn,coupled to a corresponding intensity-related measuring/detection circuit218. As mentioned above, the detection circuits 218 are triggered by thetiming electronics 216. Since the monochromatic pulse 206 arrives beforethe white illumination laser pulse 208, the stop signal generated by thetiming electronics 216 can be used to set the integration window for thedetection circuits 218. The time interval between the monochromaticreturn pulse 206 and the white illumination pulse 208 is fixed by thedifference in fiber lengths in the two transmit paths.

The narrow-band filter 212 and wavelength separation may be achievedusing a combination of conventional optics, e.g., beam-splitters andlenses. Alternately, and preferably, the optical system, aside from thetransmitter and receiver the macro-optics 210, is composed entirely ofoptical fibers and optical fiber components and devices well known tothose skilled in the art.

Regardless of the mechanism utilized, the LIDAR information and thecolor information are extracted simultaneously from the same originallaser pulse. Furthermore, the LIDAR pulses 206 and the illuminationpulses 208 are launched out of the same, preferably single-mode, opticalfiber 203 and, hence, have perfectly overlapping beam patterns orprofiles.

The full color LIDAR system 200 utilizes a single laser source andprovides multicolor/spectral resolution. It requires no separateconventional camera or alternative optical path in the out-going opticalpath and eliminates registration errors and spot or pixel sizemismatches between the return LIDAR and color information. In short, thefull color LIDAR system 200 is simplified over the conventional systemsand registration is near perfect.

As a variation on the system 200 shown in FIG. 2, a separate, second“illumination” source is coupled to the LIDAR laser beam fiber by afiber-optic coupler-splitter, i.e., sharing the essential common exitand launch into the transmitter macro-optics 210. Suitable illuminationlight sources include a combination of different wavelength (pulsed)laser diodes and super-bright white light emitting diodes (LEDs). Toensure registration, the illumination path is triggered within a shortperiod of time of the LIDAR pulse, typically a microsecond, so as toeffectively freeze the target scan motion. In this respect, theillumination source can be comparatively slow (microseconds rather than100 s picoseconds) to the advantage of not adding significantly to the(combined) accessible emission limit (AEL) and system laserclassification.

FIG. 3 shows an alternate embodiment of a full color LIDAR system 300 inaccordance with the present invention.

The full color LIDAR system 300 uses a “white” laser 302 as discussedabove with respect to the system 200 of FIG. 2. However, as shown inFIG. 3, upon return at receiver macro-optics 304, the color componentsare wavelength separated by a wavelength coupler-splitter (WDM) 306 intomultiple color (e.g. Red (R), Green (G), Blue (B)) components, each ofwhich is propagated down a separate fiber optic delay line R, G, B ofdifferent lengths. This produces a plurality of time-separated ortime-resolved wavelength-separated “color” pulse components. Thetime-resolved color pulse components are then recombined, preferablyusing a fiber-optic coupler-splitter (combiner) 308, for detection at acommon detector 310, e.g., an ADP. By synchronization with multi-triggertiming electronics 312 and associated pulse intensity measurementcircuitry (not shown), all of the spectral/color components can bedetected and their arrival time, corrected from knowledge of theindividual lengths of the fiber delay lines, recorded together withtheir relative intensity (image color).

In the system 300 of FIG. 3, a single laser and a single detector areutilized in a system that maintains spectral/color resolution. As in thesystem 200 of FIG. 2, there is no separate conventional camera oralternative optical path in the out-going optical paths and noregistration errors or spot or pixel size mismatches between returncolor and LIDAR information. Absolute scanner LIDAR accuracy ismaintained for each of the multiple color components.

In the latter respect, individual detectors, for example, could be usedfor each spectral/color component, but the result would be inevitabletiming errors between the devices due to varying performances, whichthough small, can be significant in high accuracy LIDAR systems.

In each of the full color LIDAR systems of FIG. 2 and FIG. 3, the LIDARand color or illumination beams are either one and the same, orco-propagate down the same exit fiber. Hence, they remain fullyregistered in the scanned scene or target. This provides fullyregistered LIDAR and color scan images and without a discrete auxiliary(non time-resolved) camera. The spectral/color and LIDAR information isobtained using various combinations of wavelength-division andtime-division, preferably based upon fiber optics and fiber opticscomponents and devices. The outgoing laser beam can be periodicallydirected at an internal target for LIDAR timing calibration and colorcalibration. In this way, various return LIDAR signals can be correctedfor timing drifts or differences and the laser/detective spectrumnormalized for final color rendition. Scanning can be performed not onlyin true (visible) RGB color, but also in spectroscopically-sensitivemode. In this latter context, fiber optic devices are available withwavelength splitting resolutions down to 0.1 nm, which is usefullynarrow for atomic/molecular spectroscopy. Thus, the LIDAR return fromthe “white light” laser output may be analyzed spectroscopically. Asdiscussed above, in one scheme, return LIDAR light is divided into threenarrow band wavelengths: a mid wavelength centered on a chosenabsorption line, plus two close side points as absorption intensityreferences. In this way, LIDAR measurements may be combined andperformed simultaneously with differential spectroscopy. Applicationsinclude mining and mapping of pollutants/hazardous chemicals.

Presently, effort is being made to integrate individual LIDAR deviceelements into arrays, so as to produce a “LIDAR camera,” analogous toconventional camera CCD or CMOS arrays.

With reference to FIG. 4, an embodiment of the present inventionprovides a system 400 that utilizes fiber optics technology to produce atime-division (time multiplexed) or a wavelength-division (wavelengthmultiplexed) “LIDAR array” without need of multiple lasers or detectors.

In the system 400 of FIG. 4, the output from a pulsed LIDAR laser 402 issplit into multiple paths using a fiber optic coupler-splitter 404 anddelay lines of varying length, and imaged as a time-lapse array of spotsin the scan field.

The detector's field-of-view encompasses the entire laser output, allthe laser spots being detected. The laser spots are time-separated orresolved with set (known) delays using the fiber optic delay lines. Upondetection, each individual laser spot can be resolved and detected insequence using a single optical detector 408 and multiple trigger timingelectronics 410. In brief, the laser spots are spatially-differentiatedand then time-multiplexed.

Laser imaging optics 406 may be a linear series of lenses or a 2D arrayof lenses or lenslets, depending on whether the spot array is to belinear or 2D, respectively.

FIG. 5 shows an alternate embodiment of a multiplexed full color LIDARsystem 500 in accordance with the present invention. The system of FIG.5 uses a LIDAR laser source 502 and multiplexes the receiver path, shownin FIG. 5 in linear array form.

In the case of both the system in FIG. 4 and the system in FIG. 5, whichutilize spatially separated/time-staggered laser spots or detectorimages, LIDAR measurements can be made in arrays using a single-shotlaser and a single detector. In either case, cylindrical optics can beused to produce linear arrays, and spherical optics can be used for the2D array. In both cases time, by the way of fiber optic delay lines, isused to separate the LIDAR channels. Given a white laser, however, andthe wavelength separation optics described above, wavelength can also beused to separate the channels, as shown in the embodiment of FIG. 6 of amultiplexed full color LIDAR system in accordance with the invention.

In a system 600 of FIG. 6, the laser pulse from a white laser source 602is separated by wavelength by a splitter 604. Separate portions of thespectrum are simultaneously transmitted to the target object atdifferent angular directions. The receive channel Rx then separates thereflected wavelength separated signals either into separate detectorchannels and provides them to a multiple input channel timing circuit,or, as shown in FIG. 6, time multiplexes each channel using opticaldelay lines into a single input, multiple trigger timing electronics.

There are two important advantages to the scheme of FIG. 6. First, thedata generation rate is multiplied up, being equal to the laserrepetition rate times the number of array elements. Second, laser beamarea can be extended over an area or dimension considerably wider thanthe nominal 7 mm dilated eye pupil diameter.

In the embodiment of FIG. 4, any upper laser firing frequency limit,typically 60 kHz in Nd:YAG microchip lasers due to Q-switch recoverytimes, does not limit the LIDAR data collection rate. Thus, for a 2×2array, a nominally 60 kHz LIDAR system will collect data at 4×60 kHz=240kHz rate.

In the second instance, much greater laser output pulse energies can beemitted while remaining with the Accessible Emission Limit (LaserClassification) for a given LIDAR system.

In the third instance, the ambiguity associated with determining whichLIDAR channel is resolved by wavelength, not time. Using wavelength toseparate the channels allows the LIDAR to resolve the ambiguityassociated with having multiple pulses of light in the air at the sametime. In this manner, the third instance is able to run the LIDARwithout ambiguity much faster than other instances.

Other advantages include use of existing fiber-optic components and thefact that no pixel-to-pixel LIDAR timing/distance errors are observeddue to variations in detector-to-detector performance.

The latter, in particular, is an advantage over LIDAR focal planearrays, wherein detector performance will vary from pixel-to-pixel,which will limit the ultimate timing or distance accuracy.

With reference to FIG. 7, the ability to combine optical beams with, forexample, a fiber optic coupler-splitter, allows one or more diagnosticchannels to be coupled either synchronously or asynchronously with theLIDAR path with near perfect optical registration. By sharing the sameexit or launch optical fiber, the LIDAR and auxiliary channel(s) stay inoptical registration. The auxiliary channel can be any suitable remote,fiber-based optical measurement device including temperature (IRviewing), velocity detection, vibration detection, phased LIDAR, orlaser ultrasonic or acoustic non-destructive testing.

One advantage of fiber optics over conventional macro-optics is thatmultiple optical signals can be combined and launched down a singlefiber at high efficiency. This can be applied with advantage to simplifyLIDAR systems, as in exemplified in a system block diagram of FIG. 8. Inthe system of FIG. 8, a laser is launched out of the fiber (preferablysingle_mode) network and collimated using a lens. The return beam isfocused using the same lens and launched back down the same fiber, andrerouted to the detector. The advantages of the scheme of FIG. 8 includea minimum number of components and no transmitter-receiver alignment.

In this case, the outgoing and receive beam, by sharing the same exitfiber, stay in perfect relative alignment, thus making for a simple andvery rugged LIDAR system format.

The foregoing description indicates that there are several ways ofresolving divided pulse components based upon the time that thecomponents are received by a detector. For example, as discussed abovewith respect to FIG. 2, detectors 222 receive pulse signals fordetermining spectral characteristics and the pulses received by thesedetectors 222 are associated with a pulse signal received by the LIDARdetector 214 by a known time delay caused by a delay loop 208. In FIG.3, a pulse signal for one of three (in the preferred embodiment)wavelengths is delayed by a known time, and so each of the pulse signalscan be associated with the others for each pulse emitted by the whitelaser 302. Similar principles apply in the embodiments shown in FIGS.4-6, in which known time delays are used to resolve the pulse signalsand create a single point per pulse.

Additionally, those skilled in the art will appreciate that attenuatorsand filters may be included in either the monochromatic or white lightpath to balance the illumination and ensure laser safety.

Thus, it should be recognized that a number of variations of theabove-identified embodiments will be obvious to one of ordinary skill inthe art in view of the foregoing description. Accordingly, the inventionis not to be limited by those specific embodiments and methods of thepresent invention shown and described herein. Rather, the scope of theinvention is to be defined by the following claims and theirequivalents.

What is claimed is:
 1. A method of obtaining location informationregarding an object, the method comprising: generating a laser pulse;dividing the laser pulse into a plurality of wavelength separated laserpulse portions; simultaneously directing each of the wavelengthseparated laser pulse portions to the object at a different angulardirection and detecting a corresponding plurality of reflectedwavelength separated laser pulse portions; and resolving the pluralityof reflected wavelength separated laser pulse portions to generatelocation information regarding the object.
 2. The method of claim 1,wherein the resolving step comprises providing the plurality ofreflected wavelength separated laser pulse portions to separate detectorchannels, each detector channel being connected to corresponding timingcircuitry for generating the location information.
 3. The method ofclaim 1, wherein the resolving step comprises providing the plurality ofwavelength separated laser pulse portions to separate detector channelsand time multiplexing each channel using optical delay lines ofdifferent length, the optical delay lines being connected to timingcircuitry for generating the location information.
 4. A system forgenerating both location and spectral information for a plurality ofpoints on an object, the system comprising: a laser that emits pulses oflight; means for splitting each pulse of light into a plurality of pulsecomponents of a respective plurality of wavelengths; means for detectingthe plurality of pulse components; and means for resolving the pluralityof pulse components that are associated with a single point on theobject, wherein the means for resolving corresponds the pulse componentsto each other based upon a known time delay for each pulse component. 5.The system of claim 4, wherein the plurality of pulse components of arespective plurality of wavelengths are simultaneously directed to theobject.
 6. The system of claim 5, wherein each of the plurality of pulsecomponents of a respective plurality of wavelengths simultaneouslydirected to the object at a different angular direction.
 7. The systemof claim 4, wherein the means for splitting each pulse of light into aplurality of pulse components comprises a beam splitter.
 8. The systemof claim 4, wherein the means for detecting the plurality of pulsecomponents comprises a plurality of detector channels wherein each ofthe plurality of detector channels is configured to receive one of theplurality of pulse components.
 9. The system of claim 8, wherein themeans for resolving the plurality of pulse components comprises amultiple trigger timing electronic device configured to: receive each ofthe plurality of pulse components from each of the plurality of detectorchannels, and time-multiplexing the plurality of pulse componentsreceived from each of the plurality of detector channels using opticaldelay lines of different length, wherein the optical delay lines areconnected to timing circuitry for generating the location information.